Fetal heart monitoring

ABSTRACT

An apparatus for detecting a fetal heartbeat includes circuitry to convert acoustic energy representative principally of a maternal heartbeat into a first electrical signal and circuitry to convert acoustic energy representative of a maternal heartbeat and a fetal heartbeat into a second electrical signal. The apparatus also includes circuitry to process the first and second electrical to provide an electrical signal principally representative of the fetal heartbeat and circuitry to determine pitch periods of the signal principally representative of the fetal heart beat.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following U.S. patent applications:

U.S. patent application Ser. No. 11/416,539, filed May 2, 2006, now U.S.Pat. No. 7,539,534, and entitled “Configuration for Phonography CardioHeart Monitoring;”

U.S. patent application Ser. No. 11/417,315, filed May 2, 2006 andentitled “Transducers with Acoustic Impedance Matching for PassiveCardio Monitoring;” and

U.S. patent application Ser. No. 11/417,038, filed May 2, 2006 andentitled “Passive Phonography Heart Monitor.”

BACKGROUND

This invention relates to detecting acoustic energy and in particularfetal heart monitoring.

Fetal heart monitoring is a diagnostic tool to indicate the overallhealth status of a fetus. Currently deployed fetal heart monitoringtechniques are primarily ultrasound, Doppler-based. With a typicalultrasound Doppler-based technique, wires are deployed between an ultrasound transducer unit and processing unit. A skilled operator, such as amedical technician or nurse scans or places a transceiver on the abdomenof the patient. Typically, the operator covers a region on the abdomenwith a gel and moves the ultrasonic sensor around the area to scan thearea. Alternatively, the sensor can be affixed with a belt that is wornaround the woman. The belt is cumbersome and inaccurate (often thesensor slips off of its target) and it has to be removed prior to anysurgery or emergency procedure. Acoustic signals are emitted from thetransducers and their echo signals are detected by the transceiver andprocessed to produce data pertaining to the fetal heart rate.

Current Doppler-based techniques for fetal monitoring have severallimitations. One limitation of current Doppler-based techniques is thelack of specificity for detecting fetal heart tones (FHT's). In cases ofmaternal tachycardia, the operator may not be able to differentiatewhether the transducer is detecting the fetal or maternal signal, andthis can have catastrophic consequences.

Other limitations pertain to changes in fetal position or station whichoften require re-positioning of the transducer, which can betime-consuming and result in “blackout” periods in fetal monitoring,during which medical personnel do not receive data from monitors thatmonitor the fetus. Another limitation is the loss of continuousmonitoring in a distressed fetus, especially during transition periods,e.g., moving from a delivery room to an operating room for an emergencyCesarean section procedure. In addition, many hospital protocols requiredetachment of all wires from fetal monitoring devices during roomtransfers. Detaching fetal monitors begins another “blackout period.”

Administration of epidural anesthesia presents another potential“blackout” period for fetal monitoring, as the transducer is frequentlyremoved or displaced during that procedure. This, too, is a criticaltime frame for fetal monitoring, as epidural anesthesia may causematernal hypotension with subsequent fetal bradycardia.

Maternal ambulation has been shown to facilitate labor progress, butcurrent techniques typically preclude such standing deliveries.

A newer monitoring technique known as fetal phonography uses a passiveacoustic sensor to capture acoustic energy from the maternal abdomen.Typically, the sensor includes a piezoelectric element. In a paperentitled “Development of a Piezopolymer Pressure Sensor for a PortableFetal Heart Rate Monitor” by Allan J. Zuckenvar et al., IEEETRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 40, NO. 9. SEPTEMBER 1993p. 963, the authors described a pressure sensor array mounted on a beltworn by the mother. The sensor array uses two polyvinyldene fluorideelements arranged in a bimorph structure, mechanically in series andelectrically in parallel.

SUMMARY

According to an aspect of the present invention, a method includesconverting acoustic energy representative principally of a maternalheartbeat into a first electrical signal, converting acoustic energyrepresentative of a maternal heartbeat and a fetal heartbeat into asecond electrical signal and processing the first and second electricalto provide an electrical signal principally representative of the fetalheartbeat. The method further includes determining pitch periods of thesignal principally representative of the fetal heart beat.

The follow embodiments are within the scope of the invention. The methodfurther includes converting acoustic energy representative of maternaluterine contractions into a third electrical signal. The method furtherincludes rendering the electrical signal principally representative ofthe fetal heartbeat on an output device. The method further includesdetermining principal components of determined pitch periods of thesignal principally representative of the fetal heartbeat. The methodfurther includes modulating the electrical signal principallyrepresentative of the fetal heartbeat with a signal in the audiblespectrum of human hearing.

The method further includes determining an initial period length valueof the signal principally representative of the fetal heartbeat byfinding a cepstrum of the first few pitch periods of the signalprincipally representative of the fetal heartbeat to determine thefrequency of the signal. The method further includes determining abeginning and ending point of each pitch period in the signalprincipally representative of the fetal heartbeat. The method furtherincludes determining a variation of time durations between pitch periodsand using the length of a prior period as an input to determine theduration of a subsequent pitch period. The further includes applyingprincipal component analysis to the determined pitch periods to compressdata representing the determined pitch periods. The method furtherincludes processing the determined pitch periods to provide arepresentation, compressing the representation of the determined pitchperiods, and storing the compressed representation of the determinedpitch periods.

According to a further aspect of the present invention, a computerprogram product residing on a computer readable medium for detectingfetal heartbeat energy includes instructions to convert acoustic energyrepresentative principally of a maternal heartbeat into a firstelectrical signal, convert acoustic energy representative of a maternalheartbeat and a fetal heartbeat into a second electrical signal, processthe first and second electrical to provide an electrical signalprincipally representative of the fetal heartbeat and determine pitchperiods of the signal principally representative of the fetal heartbeat.

The following are embodiments within the scope of the invention.

The computer program product further includes instructions to convertacoustic energy representative of maternal uterine contractions into athird electrical signal. The computer program product further includesinstructions to render the electrical signal principally representativeof the fetal heartbeat on an output device. The computer program productfurther includes instructions to determine principal components ofdetermined pitch periods of the signal principally representative of thefetal heartbeat. The computer program product further includesinstructions to modulate the electrical signal principallyrepresentative of the fetal heartbeat with a signal in the audiblespectrum of human hearing.

The computer program product further includes instructions to determinean initial period length value of the signal principally representativeof the fetal heartbeat by finding a cepstrum of the first few pitchperiods of the signal principally representative of the fetal heartbeatto determine the frequency of the signal. The computer program productfurther includes instructions to determine a beginning and ending pointof each pitch period in the signal principally representative of thefetal heartbeat. The computer program product further includesinstructions to determine a variation of time durations between pitchperiods and use the length of a prior period as an input to determinethe duration of a subsequent pitch period. The computer program productfurther includes instructions to apply principal component analysis tothe determined pitch periods to compress data representing thedetermined pitch periods. The computer program product further includesinstructions to process the determined pitch periods to provide arepresentation, compress the representation of the determined pitchperiods and store the compressed representation of the determined pitchperiods.

According to an additional aspect of the present invention, an apparatusincludes circuitry to convert acoustic energy representative principallyof a maternal heartbeat into a first electrical signal, circuitry toconvert acoustic energy representative of a maternal heartbeat and afetal heartbeat into a second electrical signal, circuitry to processthe first and second electrical to provide an electrical signalprincipally representative of the fetal heartbeat and circuitry todetermine pitch periods of the signal principally representative of thefetal heart beat.

The following are embodiments within the scope of the invention.

The apparatus includes circuitry to convert acoustic energyrepresentative of maternal uterine contractions into a third electricalsignal. The apparatus includes circuitry to render the electrical signalprincipally representative of the fetal heartbeat on an output device.The apparatus includes circuitry to determine principal components ofdetermined pitch periods of the signal principally representative of thefetal heartbeat. The apparatus includes circuitry to modulate theelectrical signal principally representative of the fetal heartbeat witha signal in the audible spectrum of human hearing. The apparatusincludes circuitry to determine an initial period length value of thesignal principally representative of the fetal heartbeat by finding acepstrum of the first few pitch periods of the signal principallyrepresentative of the fetal heartbeat to determine the frequency of thesignal.

The apparatus includes circuitry to determine a beginning and endingpoint of each pitch period in the signal principally representative ofthe fetal heartbeat. The apparatus includes circuitry to determine avariation of time durations between pitch periods and circuitry to usethe length of a prior period as an input to determine the duration of asubsequent pitch period. The apparatus includes circuitry to applyprincipal component analysis to the determined pitch periods to compressdata representing the determined pitch periods. The apparatus includescircuitry to process the determined pitch periods to provide arepresentation, compress the representation of the determined pitchperiods, and store the compressed representation of the determined pitchperiods.

One or more aspects of the invention may provide one or more of thefollowing advantages.

The monitor is capable of functioning without a skilled technician beingpresent. Additionally, the monitor can be relatively low in costcompared to currently employed ultrasound based monitors by avoidingneed for relatively expensive crystals commonly employed in theultrasound transducers. The monitor uses low-cost sensing, transmission,and circuitry components suitable for operation in hospitals, physicianoffices, or home environments.

The monitor uses transducer sensor units that are disposable. Thedisposable nature of the transducer sensor units enables the monitor toensure a very high standard of accuracy for these transducer sensorunits because the term of use for each transducer sensor unit will notexceed a specified time duration. Hence, normal concerns of qualitydegradation resulting from extended use are avoided, while maintaining arelatively high level of performance. The monitor avoids blackoutperiods, e.g., the potentially most dangerous window of time duringlabor since the monitor in the wired and especially the wireless formallows for constant monitoring. Accurate, wireless monitoring systemaids in decreasing labor time by increasing the potential mobility ofthe patient, thus making the resources in a labor-and-delivery unit moreavailable.

The monitor uses a pitch period detector, a principal componentanalyzer, and a complex wavelet filter bank to analyze signals from thesensors. This permits sophisticated and accurate fetal signal processingto be employed in the monitor at a relatively low cost. The monitorallows for maternal ambulation during labor, providing a number ofpotential benefits.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a monitoring scheme.

FIG. 2 is a block diagram of fetal monitor device used to monitor fetalcardiac activity.

FIG. 3 is a flow chart depicting aspects of processing in the fetalmonitoring device of FIG. 2.

FIG. 4 is a block diagram of an alternative fetal monitor device.

FIG. 5 is a block diagram depicting processing.

FIGS. 6A-6E-8A-8C (collectively, FIGS. 6-8) are diagrams depictingconstruction details of sensors used with the monitor of FIG. 3.

FIGS. 9A-9B (collectively FIG. 9) is a set of diagrams depicting analternate pattern for a piezoelectric sensor element.

FIG. 10 is a block diagram of circuitry used in the sensors.

FIG. 11 is a schematic of a high impedance amplifier used with thesensors of FIGS. 6-8.

FIG. 12 is a block diagram depicting details of pitch processing

FIG. 13 is a flow chart depicting pitch processing.

FIGS. 14A and 14B are diagrams useful in understanding processing offetal and maternal heartbeat signals.

FIG. 15 is a flow chart depicting principal component analysis.

DETAILED DESCRIPTION

Referring to FIG. 1, an arrangement 10 for connection of a monitordevice 12 (“monitor”) to a patient, e.g., pregnant woman 14 to monitorfetal heartbeat signals is shown. The monitor 12 can be used for varioustypes of monitoring, as discussed below. In this example, the monitor 12is a fetal heartbeat monitor.

The monitor 12 (discussed in detail below) has acoustic transducer(sensors) 16 a-16 c that convert acoustic energy from the pregnant woman14 into electrical energy. The transducers 16 a-16 c are coupled to themonitor 12, via communication channels, 18 a-18 c, which can be wiresconnecting to the monitor 12 or wireless channels (radio frequency,optical and/or infrared). In one embodiment, Bluetooth® wirelesstechnology is used.

In one configuration for connection of the monitor 12 to the patient,one of the transducers, e.g., transducer 16 a monitors the pregnantwoman's heartbeat, another one of the transducers 16 b monitors thepregnant woman's uterus to measure uterine contractions. The transducerto monitor the uterine contractions, is not essential to capturing thefetal heartbeat but is included as part of an overall tool to monitorthe health and status of the patient and fetus. The third transducer 16c monitors the fetal heartbeat. The location of the pregnant woman'sheart and uterus are readily predictable. The acoustic energy from thefetal heart is omni-directional but localized about the back of thefetus. Such localization is attributed to preferred acoustic propagationto sites where the fetal back is against the maternal abdominal wall.The acoustic propagation through the maternal wall is omni-directionalbut there is a point of maximum acoustic conduction, which is the pointwhere the fetus' back is pressed against the uterine wall. However,other positions can be used to attach the transducer 16 c to thepregnant woman.

In another configuration for connection of the monitor 12 to thepatient, transducer 16 a is arranged to monitor the pregnant woman'sheartbeat and transducers 16 b monitors the pregnant woman's uterus tomeasure uterine contractions. To capture fetal acoustic energy, aplurality of transducers (not shown) 16 c can be deployed to monitor thefetal heartbeat. The multiple acoustic transducer 16 c are deployed forfetal detection and arranged about the maximal fetal acoustic energy.This is a noise reduction technique that can be used in cases where itis difficult to sense the fetal heartbeat (e.g., in the case of anoverweight pregnant woman or underweight fetus) extra fetal sensors canbe deployed to boost the strength of the fetal signal. Furthermore, 3 ormore fetal sensors can be used to triangulate the position of the fetalheart. This localization information can be used by doctors andtechnicians during labor and delivery.

Referring to FIG. 2, the monitor 12 includes a processor 30, e.g., ageneral purpose central processing unit (CPU) and/or a digital signalprocessor (DSP) to process signals from the patient, a memory 32, toexecute programs, persistent, e.g., non-volatile storage 34, and I/Ointerface(s) 36 all coupled via a bus 38. Executed by the monitor 12 issignal processing software 50 that processes ECG signals detected bytransducers 14 a and 14 c from the pregnant woman's heart and thefetus's heart, respectively. The monitor 12 also processes signals fromthe transducer 14 b that monitors for contractions in the pregnantwoman's uterus.

Processing 50 provides a relatively clean detection of the fetalheartbeat by eliminating major sources of noise in the fetal heartbeatsignal, e.g., the relatively strong acoustic energy componentscontributed to the detected fetal heartbeat caused by the pregnantwoman's heartbeat. In some embodiments, acoustic energy components fromuterine contractions could also be filtered from the detected fetalheartbeat acoustic energy, but in general that is an insignificantcontributor to noise in detection of the fetal heartbeat.

The monitor 10 can also include other user interface devices, e.g.,keyboard or keypad, a display, speakers, headphone, etc. (not shown). Inaddition, the monitor can include a transmission channel to upload datato a server or the like.

Referring to FIG. 3, the monitor 12 includes an interface 36 thatinterfaces the monitor 12 to the transducers 16 a-16 c. The interface 36here is shown to include channels 36 a-36 c for transducers 16 a-16 c,respectively. Each channel 36 a-36 c includes a receiver 40 (if themonitor is a wireless version) or an analog signal interface (not shown)to cables (not shown) from the transducer, if the monitor 12 is awire-connected version. In addition, the interface 36 includes a lownoise amplifier and a filter generally 42 to process analog signals fromthe transducers 16 a-16 c.

The amplifier 14 amplifies the signals and the filter filters thesignals to preserve frequencies in the range of, e.g., 0.05 to 100 Hz orso. Typically, the fetal channel in the monitor 12 can be within thebroad range above, but most likely will in a range about 10 to 30 Hz andespecially in a range of 18 to 25 Hz (the range of maximal spectralpower of the fetal heart signal). The maternal channel can be within thebroad range above, but most likely will in a range about 6 to 14 Hz andespecially in a range of 8 to 12 Hz (the region of maximal power of thematernal heart signal). Whereas, the transducer 14 b that senses thematernal contractions need not have any filtering since it is a verylong period, e.g., a large impulse.

Each amplifier 14 feeds the signal to an A/D converter 44 that digitizesthe signal, at a sampling frequency at least greater than twice thehighest frequency component in the channel. In other implementations, asingle A/D converter and a multiplexer can be used to process data fromthe channels (See FIG. 4). The digitized signals from each of thechannels are transferred to the bus interface device 46 that formats thedigitized signals to place on the bus 38 (FIG. 2) to send to the memory34 and/or processor 32 to be processed.

Referring to FIG. 4, an alternative arrangement for the monitor 12interfaces the monitor 12 to the transducers 16 a-16 c. A channel 36a-36 c is provided for each transducer 16 a-16 c. Each channel 36 a-36 cincludes a receiver 40 (if the monitor is a wireless version) or ananalog signal interface (not shown) to cables (not shown) from thetransducer, if the monitor is a wire-connected version. In addition, theinterfaces 36 a to 36 c include a low noise amplifier and a filtergenerally 42 to process analog signals from the transducers 16 a and 16c and a low noise amplifier generally 42′ to process analog signals fromthe transducer 16 b.

The amplifier 14 amplifies the signals and the filter filters thesignals to preserve frequencies in the ranges discussed above. Eachamplifier/filter 42 and amplifier 42′ selectively feeds its outputsignal to a A/D converter/multiplexer 44 that digitizes the signal, at asampling frequency at least greater than twice the highest frequencycomponent in the channel, according to control provided from theprocessor. The single A/D converter and multiplexer 44 processes data inthe selected channel and transfers the data to the digital signalprocessor 45 (DSP) for processing described below.

A processor 48 processes signals from a front panel to control theADC/mux 44, whereas the DSP 45 processes output signals from the ADC/mux44 to provide outputs to the front panel. In some implementations thiscan be the same device. The front panel thus includes a display, adigital readout, switches (to select which channel to process),speakers, and so forth. The monitor 10 can also include other userinterface devices, e.g., keyboard or keypad, and interfaces forconnection to other equipment to upload data to a server and the like.

The arrangement also includes memory, to execute programs, persistent,e.g., non-volatile storage, and I/O interface(s) all coupled via buses(not shown) to the digital signal processor 45 and processor 48.

Executed by DSP 45 is signal processing software 50 that processessignals from the transducers 16 a and 16 c from the pregnant woman'sheart and the fetus's heart, respectively. The monitor also processessignals from the transducer 16 b that monitors for contractions in thepregnant woman's uterus. This data are fed to the processor to determinecontraction rates that are sent to the front panel for display.

Processing 50 provides a relatively clean detection of the fetalheartbeat by eliminating major sources of noise in the fetal heartbeatsignal, e.g., the relatively strong acoustic energy componentscontributed to the detected fetal heartbeat caused by the pregnantwoman's heartbeat. In some embodiments, acoustic energy components fromuterine contractions could also be filtered from the detected fetalheartbeat acoustic energy.

Referring to FIG. 5, processing of signals from the transducers isshown. The signals from channels 36 a, 36 c are passed through digitalband pass filters 51 a, 51 b to filter the signals in the rangediscussed above, e.g., 18 to 25 Hz for the fetal channel and 8 to 12 Hzfor the maternal channel. The other ranges above could be used.

The component of the pregnant woman's heartbeat that appears in thefetal channel is removed from the fetal signal in the difference block51 c. From the difference block, the signal is fed to a pitch trackprocessor 52. The pitch track processor 52 uses pitch tracking and aprincipal component analysis to generate waveforms that can be used todetermine heart rates, e.g., in heart rate processor 55 and process thesignal to provide an ECG from ECG processor 56. These signals can bedisplayed on display 58.

The modulator 54 takes the output signal from the difference block 51 dand modulates it with a signal in the audible spectrum of human hearing.That is, the modulator adds a carrier to the signal from the differenceblock 51 d to provide an output signal that can be heard by humans. Thissignal can be converted to an analog representation and fed to an audioamplifier, to be rendered from a speaker 58 b, etc. Details ofprocessing are discussed below.

Referring to FIGS. 6A-6E through 8A-8C, collectively FIGS. 6-8, detailsof construction for an acoustic transducer “button” 16 c transducer toacquire sound waves in the audible spectrum from the fetal heart areshown. A similar arrangement can be used for the transducer 16 a toacquire the maternal heart beat signal and transducer 16 b, thetocodynamometer (TOCO) transducer to detect maternal contractions, asfurther described below.

Transducer 16 c is a relatively small, self-adhering, device that, insome implementations, is wireless. Transducer 16 c is attached to theepidermis of the maternal abdomen, via a layer of an adhesive, e.g., anadhesive tape 61, in particular a double-sided adhesive, which inaddition to providing for attachment of the transducer 16 c to theepidermis also provides acoustic impedance matching between theepidermis and a piezoelectric membrane that detects acoustic energy inthe transducer. The transducer 16 c captures acoustic energy thatemanates from the maternal abdomen through the uterus.

Referring to FIGS. 6A-6E, collectively, FIG. 6, the acoustic transducer“button” 16 c includes a base member 60. The base member 60, as depictedin FIG. 6A, includes a frame arrangement 62 that supports bosses 64 tocarry a circuit board (not shown) that supports signal preconditioningcircuits, as discussed in FIG. 9.

FIG. 6A depicts an aperture 66 in a bottom portion 60 a of the base 60.A polymer membrane 68 covers a substantial portion of the aperture 66 a.The polymer membrane 68 is sandwiched between a pair of electrodes overthe opposing major surfaces of the polymer membrane 68. A pair of wires(not shown), for example, are attached to the electrodes of the polymer68. Bosses are provided in the base 60 to elevate a circuit board abovethe plane of the bottom of the base 60 to provide clearance for wires,that couple to the electrodes on the polymer membrane 68.

As shown in FIG. 6B, the polymer membrane 68 is disposed through acavity 65 in the bottom of the base 60, such that the polymer membrane68 rests within but is not interfered with by sides of the base 60 thatform cavity 65. The cavity can be eliminated. For instance, depending onmanufacturing constraints other configurations such as connecting thePCB to the membrane via electrodes provided through the base may bepreferred. In addition a foam type material can occupy the cavity, e.g.,the cavity can be filled with another material, e.g., an acoustic foammaterial. The polymer membrane 68 has a major surface that is contactedby the double-sided adhesive tape 61 on what will be the outside of thebase 60, as shown in FIG. 6C, and a second major surface that is withinthe transducer.

The adhesive layer 61 is provided on the bottom of the base and over theoutside surface of the polymer membrane 68. In general, the adhesivelayer contacts the polymer membrane 68 on the outside, major surface,thus securing the polymer membrane 68 into the transducer. The adhesive69 is provided as a double-sided adhesive medical-grade tape of a 4.5mil double coated polyester tape, coated on both sides with ahypoallergenic, pressure sensitive synthetic rubber based adhesive on a1 mil transparent polyester carrier, with a release liner siliconecoated 60 1 b bleached Kraft paper. This tape is ethylene oxide, gammaand autoclave process tolerant. One suitable product is Tape No. 9877from 3M Corporation Minneapolis Minn. Other adhesive tapes and adhesivescould be used.

In conventional approaches, as mentioned above an acoustic match isprovided by a gel that is applied on the maternal abdomen. Typically,the operator covers a region of the abdomen with the gel (a slippery,non-sticky clear gel) and moves the ultrasonic sensor around the area toscan the area. Alternatively, the conventional ultrasonic sensor can beaffixed with a belt that is worn around the woman. The belt iscumbersome and especially inaccurate (since often the sensor slips offof its target) and it has to be removed prior to surgery or emergencyprocedures.

In contrast, the adhesive tape 61 secures the polymer membrane to thetransducer 16 a, holding one major surface of the polymer, e.g., theouter surface of the polymer, while permitting the other major surfaceof the polymer 68 to be free to vibrate in the cavity 65 of thetransducer. The adhesive tape 61, as discussed above, provides acousticcoupling between the polymer 68 and the maternal abdomen. In someembodiments, material can be interposed between the tape and the polymermembrane for additional acoustic impedance matching. Here the tape 69provides acoustic impedance matching, while securing the polymer 68 tothe transducer 16 c and also securing the transducer 16 c to the abdomenof the patient.

As depicted in FIG. 6D, a snap member 71 is disposed on an inner portionof the sidewall of the base member 60, to fasten a dome cap member 74(FIGS. 7A-7D) to the base member 60. Here five additional snap membersare disposed about the base, adjacent to the bosses, as denoted by “S.”FIG. 6E shows a side view of the base member 60 from a side opposing theslot 69.

Referring to FIGS. 7A-7D, collectively FIG. 7, the dome cap member 80 isillustrated. The dome cap 80 has a generally convex outer surface, asdepicted in FIG. 7A. The dome cap member supports a set of binding posts82 that align with the base member 80 (FIG. 6) to secure the circuitboard (not shown) inside the dome cap 80 and urge the circuit boardagainst the bosses 64 on the base member 60, as depicted in FIG. 7C. Thedome cap 80 has a generally convex outer surface to increase themechanical integrity of the transducer housing.

FIGS. 7C and 7D depict details of the snap receptacle member 84 tosecure the dome 80 to the base 60. Other fastening arrangements arepossible including gluing, screw fastening, welding and so forth.

The base 60 and the dome 80 are comprised of a generally translucentmaterial. One type of material for the dome 80 and base 60 is ABS,especially medically approved ABS. ABS is a plastic, especially any of aclass of plastics based on acrylonitrile-butadiene-styrene copolymers.ABS has sufficient strength to support the weight of a pregnant womenshould she roll over onto the transducer, is medically approved, and istranslucent. Other types of materials, especially plastics havingsufficient strength and preferably translucence or transparency could beused.

By using a translucent (or transparent) plastic, an optical type ofindicator, such as a light emitting diode (LED) can be coupled to thecircuitry inside the device. One or a series of LED's can be used toindicate status and health of the transducer, as discussed below. TheLED's could also be outside of or mounted into the base or dome thedevice.

Referring to FIGS. 8A-8D, the assembled transducer 16 c is illustratedwith the base member 60 secured in place to the dome cap 80, with thepolymer membrane 68 exposed on the bottom with the adjacent cavity 66.

Referring to FIGS. 9A-9B, collectively FIG. 9, an alternativeconstruction is shown. Here the base member 60′ has a aperture 66′ thatis in a generally “Y” shape, e.g., with three rectangular apertureregions converging together, in which are disposed three (3) polymermembranes 68 a-68 c. The membranes 68 a-68 c improve sensitivity and canbe electrically coupled in series to increase the overall voltageproduced from the patient or in parallel to increase the amount ofcharge and hence reduce the input impedance for the high impedanceamplifier.

The polymer membrane 68 or 68 a-68 c can be comprised of any suitablepolymer material that exhibits piezoelectric properties. Certain polymerand copolymer materials such as polyvinyldene fluoride (PVDF) have longrepeating chains of “CH₂—CH₂” molecules that when “orientated” provide acrystalline structure and a net polarization. Such a sheet of orientatedmaterial disposed between a pair of electrodes, for example, can detectmechanical energy by producing a net charge or produce mechanical energyby application of charge.

Films can be obtained from Measurement Specialties Inc. Valley Forge Pa.as part No. SDT1-028k, which is equivalent to DT1-028k whose propertiesare in the table below, but without a protective urethane coating. Thisis a 028 micron thick polymer sheet with Silver ink electrodes althoughNiCu-alloys could be used. Leads can be placed on separately or can beprovided by the manufacturer. Leads can be attached by compressiveclamping, crimps, eyelets, conductive epoxy or low temperature soldersand so forth.

B D E F A elec- C elec- thick- Capaci- Number Film trode film trode nesstance DT1- .64 (16) .484 (12) 1.63 (41) 1.19 (30) 40 1.38 nf 028K

Where dimensions A-E are in millimeters (mm), F is capacitance (nf)nanofarads and where A and C are the width and length of the film, B andD are the width and length of the electrode and E is the thickness ofthe PVDF polymer. Other thickness, sizes and types of piezoelectric PVDFpolymer could be used.

In one mode of operation, mechanical energy in the form of acousticenergy from the pregnant woman (detected fetal and maternal heartbeatsor detected contractions) impinge upon the combination of electrodes andsheet of material causing mechanical deforming of the orientatedcrystalline structure of the sheet. This mechanical deformation producesa voltage potential across the sheet of material, providing a potentialdifference between the pair of electrodes. This potential difference isamplified by the circuitry on the circuit board, is preprocessed, andtransmitted to the monitor 12.

The transducer 16 a for measurement of audible spectrum sound waves fromthe maternal heart can be constructed in a similar manner. This buttonwill be attached to the epidermis, e.g. the precordium, and will senseacoustic waves and send the signal to the interface 36 for processing.In general, the precordium is the external surface of the body overlyingthe heart and stomach, typically, in the case of a pregnant woman, underthe left breast of the patient.

A tocodynamometer (TOCO) transducer 16 b for measurement of maternaluterine contractions is also constructed in a similar manner. Thetocodynamometer (TOCO) transducer 16 b like the other transducers is aself-powered device, at least in wireless applications. Thetocodynamometer (TOCO) transducer 16 b is a small, self-adhering devicethat detects contractions of the muscles of the pregnant woman's uterusby sensing tightening of the maternal epidermis in the vicinity of theuterus. Transducer 16 b is similar in construction to the transducers 16a and 16 c, and is coupled to the monitor, via one of the inputchannels. The signal from the transducer 16 b is processed to provide ameasure of the rate of contractions of the uterus.

In an alternative embodiment, the TOCO transducer 16 b is a conventionalstrain gauge, which does not require the acoustic equipment of the heartbeat monitor.

Together, transducers 16 a and 16 c comprise a transducer system forcapturing acoustic energy that can include the fetal heart signal andwith the analysis described in FIGS. 4 and 5 can produce an audible andacoustic signal of the fetal heart from which the fetal condition can beascertained.

In addition, the transducer 16 a and 16 b provide a transducer systemthat provides signals that when processed provide an indication of thelabor status of the pregnant woman, e.g., heart rate and rate of uterinecontractions.

The set of transducers 16 a-16 c provides minimal discomfort to thepregnant woman, complete transparency with regard to the currentlyemployed delivery room fetal monitoring techniques, and minimal andvirtually no interference with emergency surgical procedures such asemergency cesarean section, especially with the wireless embodiments.

The wireless communication employed is low-power radio-frequency (RF)signals in compliance with FCC regulations posing no risk (according tocontemporary medical views) to the pregnant woman, the infant, or anytechnicians and clinicians. One preferred wireless technology employedis low power, Bluetooth® (Bluetooth® SIG, Inc.) wireless technologyapproved for medical applications.

Referring to FIG. 10, circuitry 100 on the circuit board housed in thetransducer 16 c is shown. The circuitry 100 includes a high impedanceamplifier 102 that interfaces to wires from the electrodes on thepolymer membrane 68, as well as a battery 104 and a transmitter device106 (or a analogy driver circuit (not shown) if the transducer 16 c iscoupled to the monitor 12 via wires. Also included is an antenna element108, here a dipole antenna internal to the transducer. An on-chipantenna device may also be used. Other techniques could be used such asinfrared or optical.

In a wired implementation, power to the devices could be delivered viawires that attached to the transducer, whereas in the wirelessimplementation power is provided by a small battery, as shown in FIG.10.

In one wireless implementation each transducer includes a unique deviceidentifier code 105. In operation, each transducer 16 a-16 c whenpowered up would first be registered with the monitor 12, e.g., aprocedure that stores in the monitor 12 the unique identifier of thetransducer that the monitor is wireless coupled to. Each time thetransducer sends data to the monitor, the transducer includes thetransducer identifier, so that the monitor would be certain that it isprocessing data from the correct transducer, registered for thatmonitor, and not from transducers registered with a different monitorand on a different patient.

The circuitry also includes LEDS, here three being shown that light upto indicate various statuses of the transducer. For instance, using thesituation of wireless transducers, the three LEDS, one red, one yellowand one green, can be used to indicate the statuses of respectively,“failure”, e.g., of a battery, as shown or by failing to receive anyoutput signal from the transmitter; “ready but not registered” bysensing a signal from the transmitter, which would be in that case atransceiver, which would receive a signal back from the monitorindicating that it is registered with the monitor; and “working” bysensing the output the transmitter. Alternatively, the LEDs can senseoutputs from the amplifier.

Referring to FIG. 11, the high impedance amplifier 102 is used tointerface with the polymer sheet 68. Since the polymer sheet 68 iscapacitive in nature, a high input impedance amplifier is used toamplify the voltage potential generated across the polymer sheet priorto transmission (either wirelessly or with wires) to the monitor. Thehigh impedance amplifier 102 has components to set the operating pointof the high impedance amplifier 102. The high impedance amplifier 102includes an operational amplifier 104 having differential inputs one ofwhich receives a portion of the output signal fed back to the invertinginput −INA of the amplifier 104. The signal from the sheet 68 is fed tothe non-inverting input +INA.

Referring now to FIG. 12, details of the pitch processing block 52 areshown. From the difference block, 51 d (FIG. 5) the signal is fed topitch track analyzer 120, a switch 122, a principal component analysis(PCA) generator 124 and a spacing coefficient generator 126.

Principal component analysis (PCA) is a linear algebraic transform. PCAis used to determine the most efficient orthogonal basis for a given setof data. When determining the most efficient axes, or principalcomponents of a set of data using PCA, a strength (i.e., an importancevalue called herein as a coefficient) is assigned to each principalcomponent of the data set.

The pitch track analyzer 120 determines the pitch periods of the inputwaveform. The signal switch 122 routes the signal to the PCA generator124 during an initial calibration period. PCA generator 124 calculatesthe principal components for the initial pitch period received. PCAGenerator 124 sends the first, e.g., 6 principal components for storage130 and/or further processing. After the initial period, switch 122routes the signal from the difference block to coefficient generator126, which generates coefficients for each subsequent pitch period.Instead of sending the principal components, only the coefficients aresent, thus reducing the number of bits.

Switch 16 includes a mechanism that determines if the coefficients beingused are valid. Coefficients deviating from the original coefficients bymore than a predetermined value are rejected and new principalcomponents and hence new coefficients are determined.

The pitch tracking analyzer 120 and the other components mention aboveare described in U.S. patent application Ser. No. 10/624,139 filed Jul.21, 2003, published US-2004-0102965-A1 May 27, 2004 by Ezra J. Rapoportincorporated herein by reference in its entirety.

The pitch track analyzer 120 determines the pitch periods of the inputwaveform. The pitch track analyzer 120 determines trends in the slightchanges that modify a waveform across its pitch periods includingquasi-periodic waveforms like heartbeat signals. In order to analyze thechanges that occur from one pitch period to the next, a waveform isdivided into its pitch periods using pitch tracking process 53 (FIG.13).

Referring now also to FIG. 13 a pitch tracking process 121 receives 121a an input waveform 75 (FIG. 14A) from difference block 51 c todetermine the pitch periods. Even though the waveforms of fetalheartbeat are quasi-periodic, a fetal heartbeat still has a pattern thatrepeats for the duration of the input waveform 75. However, eachiteration of the pattern, or “pitch period” (e.g., PP₁) varies slightlyfrom its adjacent pitch periods, e.g., PP₀ and PP₂. Thus, the waveformsof the pitch periods are similar, but not identical, thus making thetime duration for each pitch period unique.

Since the pitch periods in a waveform vary in time duration, the numberof sampling points in each pitch period generally differs and thus thenumber of dimensions required for each vectorized pitch period alsodiffers. To adjust for this inconsistency, pitch tracking analyzer 120designates 121 b a standard vector (time) length, V_(L). After pitchtracking process 121 executes, the pitch tracking analyzer 120 choosesthe vector length to be the average pitch period length plus a constant,e.g., 40 sampling points. This allows for an average buffer of 20sampling points on either side of a vector. The result is that allvectors are a uniform length and can be considered members of the samevector space. Thus, vectors are returned where each vector has the samelength and each vector includes a pitch period.

Pitch tracking process 121 also designates 121 c a buffer (time) length,B_(L), which serves as an offset and allows the vectors of those pitchperiods that are shorter than the vector length to run over and includesampling points from the next pitch period. As a result, each vectorreturned has a buffer region of extra information at the end. Thislarger sample window allows for more accurate principal componentcalculations (discussed below). In the interest of storage reduction,the buffer length may be kept to between 10 and 20 sampling points(vector elements) beyond the length of the longest pitch period in thewaveform.

At 8 kHz, a vector length that includes 120 sample points and an offsetthat includes 20 sampling units can provide optimum results.

Pitch tracking process 121 relies on the knowledge of the prior periodduration, and does not determine the duration of the first period in asample directly. Therefore, pitch tracking process 121 determines 121 dan initial period length value by finding a real “cepstrum” of the firstfew pitch periods of the heartbeat signal to determine the frequency ofthe signal. A cepstrum is an anagram of the word “spectrum” and is amathematical function that is the inverse Fourier transform of thelogarithm of the power spectrum of a signal. The cepstrum method is astandard method for estimating the fundamental frequency (and thereforeperiod length) of a signal with fluctuating pitch.

A pitch period can begin at any point along a waveform, provided it endsat a corresponding point. Pitch tracking process 121 considers thestarting point of each pitch period to be the primary peak or highestpeak of the pitch period.

Pitch tracking process 121 determines 121 e the first primary peak 77.Pitch tracking process 121 determines a single peak by taking the inputwaveform, sampling the input waveform, taking the slope between eachsample point and taking the point sampling point closest to zero. Pitchtracking process 121 searches several peaks within an expectation rangeand takes the peak with the largest magnitude as the subsequent primarypeak 77. Pitch tracking process 121 adds 121 f the prior pitch period tothe primary peak. Pitch tracking process 121 determines 121 g a secondprimary peak 81 locating a maximum peak from a series of peaks 79centered a time period, P, (equal to the prior pitch period, PP₀) fromthe first primary peak 77. The peak whose time duration from the primarypeak 77 is closest to the time duration of the prior pitch period PP₀ isdetermined to be the ending point of that period (PP₁) and the startingpoint of the next (PP₁). The second primary peak is determined byanalyzing three peaks before or three peaks after the prior pitch periodfrom the primary peak and designating the largest peak of those peaks asthe second peak 82.

Process 121 vectorizes 121 i the pitch period. Pitch tracking processor120 makes 121 j the second primary peak the first primary peak of thenext pitch period and recursively executes, e.g., back to 121 f,returning a set of vectors. That is, pitch tracking process 120designates 121 j the second primary peak as the first primary peak ofthe subsequent pitch period and reiterates (121 f)-(121 j).

Each set of vectors corresponds to a vectorized pitch period of thewaveform. A pitch period is vectorized by sampling the waveform overthat period, and assigning the i^(th) sample value to the i^(th)coordinate of a vector in Euclidean n-dimensional space, denoted by

^(n), where the index i runs from 1 to n, the number of samples perperiod. Each of these vectors is considered a point in the space

^(n).

FIG. 14B shows an illustrative sampled waveform of a pitch period. Thepitch period includes 82 sampling points (denoted by the dots lying onthe waveform) and thus when the pitch period is vectorized, the pitchperiod can be represented as a single point in an 82 (orhigher)—dimensional space.

Thus, pitch tracking processor 120 identifies the beginning point andending point of each pitch period. Pitch tracking processor 120 alsoaccounts for the variation of time between pitch periods. This temporalvariance occurs over relatively long periods of time and thus there areno radical changes in pitch period length from one pitch period to thenext. This allows pitch tracking process 62 to operate recursively,using the length of the prior period as an input to determine theduration of the next.

Pitch tracking processor 120 can be stated as the following recursivefunction:

${f\left( {p_{prev},p_{new}} \right)} = \left\{ \begin{matrix}{{{f\left( {p_{new},p_{next}} \right)}\text{:}{{s - {d\left( {p_{new},p_{0}} \right)}}}} \leq {{s - {d\left( {p_{prev},p_{0}} \right)}}}} \\{{{d\left( {p_{prev},p_{0}} \right)}\text{:}{{s - {d\left( {p_{new},p_{0}} \right)}}}} > {{s - {d\left( {p_{prev},p_{0}} \right)}}}}\end{matrix} \right.$

The function f(p,p′) operates on pairs of consecutive peaks p and p′ ina waveform, recurring to its previous value (the duration of theprevious pitch period) until it finds the peak whose location in thewaveform corresponds best to that of the first peak in the waveform.This peak becomes the first peak in the next pitch period. In thenotation used here, the letter p subscripted, respectively, by “prev,”“new,” “next” and “0,” denote the previous, the current peak beingexamined, the next peak being examined, and the first peak in the pitchperiod respectively. The value “s” denotes the time duration of theprior pitch period, and d(p,p′) denotes the duration between the peaks pand p′.

B. Principal Component Analysis

Principal component analysis is a method of calculating an orthogonalbasis for a given set of data points that defines a space in which anyvariations in the data are completely uncorrelated. PCA can be used as acompression technique to store pitch periods from the pitch trackingprocessor for detailed analysis. The symbol, “

^(n)” is defined by a set of n coordinate axes, each describing adimension or a potential for variation in the data. Thus, n coordinatesare required to describe the position of any point. Each coordinate is ascaling coefficient along the corresponding axis, indicating the amountof variation along that axis that the point possesses. An advantage ofPCA is that a trend appearing to span multiple dimensions in

^(n) can be decomposed into its “principal components,” i.e., the set ofeigen-axes that most naturally describe the underlying data. Byimplementing PCA, it is possible to effectively reduce the number ofdimensions. Thus, the total amount of information required to describe adata set is reduced by using a single axis to express several correlatedvariations.

For example, FIG. 6A shows a graph of data points in 3-dimensions. Thedata in FIG. 6B are grouped together forming trends. FIG. 6B shows theprincipal components of the data in FIG. 6A. FIG. 6C shows the dataredrawn in the space determined by the orthogonal principal components.There is no visible trend in the data in FIG. 6C as opposed to FIGS. 6Aand 6B. In this example, the dimensionality of the data was not reducedbecause of the low-dimensionality of the original data. For data inhigher dimensions, removing the trends in the data reduces the data'sdimensionality by a factor of between 20 and 30 in routine speechapplications. Thus, the purpose of using PCA in this method ofcompressing speech is to describe the trends in the pitch-periods and toreduce the amount of data required to describe speech waveforms.

Referring to FIG. 15, principal components process 124 determines (152)the number of pitch periods generated from pitch tracking process 121.Principal components process 124 generates (154) a correlation matrix.

The actual computation of the principal components of a waveform is awell-defined mathematical operation, and can be understood as follows.Given two vectors x and y, xy^(T) is the square matrix obtained bymultiplying x by the transpose of y. Each entry [xy^(T)]_(i,j) is theproduct of the coordinates x_(i) and y_(j). Similarly, if X and Y arematrices whose rows are the vectors x_(i) and y_(j), respectively, thesquare matrix XY^(T) is a sum of matrices of the form [xy^(T)]_(i,j):

${XY}^{T} = {\sum\limits_{i,j}{x_{i}y_{j.}^{T}}}$

XY^(T) can therefore be interpreted as an array of correlation valuesbetween the entries in the sets of vectors arranged in X and Y. So whenX=Y, XX^(T) is an “autocorrelation matrix,” in which each entry[XX^(T)]_(i,j) gives the average correlation (a measure of similarity)between the vectors x_(i) and x_(j). The eigenvectors of this matrixtherefore define a set of axes in

^(n) corresponding to the correlations between the vectors in X. Theeigen-basis is the most natural basis in which to represent the data,because its orthogonality implies that coordinates along different axesare uncorrelated, and therefore represent variation of differentcharacteristics in the underlying data.

Principal components process 124 determines (156) the principalcomponents from the eigenvalue associated with each eigenvector. Eacheigenvalue measures the relative importance of the differentcharacteristics in the underlying data. Process 124 sorts (158) theeigenvectors in order of decreasing eigenvalue, in order to select theseveral most important eigen-axes or “principal components” of the data.

Principal components process 124 determines (160) the coefficients foreach pitch period. The coordinates of each pitch period in the new spaceare defined by the principal components. These coordinates correspond toa projection of each pitch period onto the principal components.Intuitively, any pitch period can be described by scaling each principalcomponent axis by the corresponding coefficient for the given pitchperiod, followed by performing a summation of these scaled vectors.Mathematically, the projections of each vectorized pitch period onto theprincipal components are obtained by vector inner products:

$x^{\prime} = {\sum\limits_{i = 1}^{n}{\left( {e_{i} \cdot x} \right){e_{i}.}}}$

In this notation, the vectors x and x′ denote a vectorized pitch periodin its initial and PCA representations, respectively. The vectors e_(i)are the ith principal components, and the inner product e_(i)·x is thescaling factor associated with the ith principal component.

Therefore, if any pitch period can be described simply by the scalingand summing the principal components of the given set of pitch periods,then the principal components and the coordinates of each period in thenew space are all that is needed to reconstruct any pitch period andthus the principal components and coefficients are the compressed formof the original heartbeat signal. In order to reconstruct any pitchperiod of n sampling points, n principal components are necessary.

In the present case, the principal components are the eigenvectors ofthe matrix SS^(T), where the ith row of the matrix S is the vectorizedith pitch period in a waveform. Usually the first 5 percent of theprincipal components can be used to reconstruct the data and providegreater than 97 percent accuracy. This is a general property ofquasi-periodic data. Thus, the present method can be used to findpatterns that underlie quasi-periodic data, while providing a concisetechnique to represent such data. By using a single principal componentto express correlated variations in the data, the dimensionality of thepitch periods is greatly reduced. Because of the patterns that underliethe quasi-periodicity, the number of orthogonal vectors required toclosely approximate any waveform is much smaller than is apparentlynecessary to record the waveform verbatim.

Another type of analysis is the complex wavelet transform, as describedin Dual-Tree Complex Wavelet Transform, Ivan W. Selesnick, et al., IEEESignal Processing Magazine 123 November 2005, which is incorporatedherein in its entirety.

The invention can be implemented in digital electronic circuitry, or incomputer hardware, firmware, software, or in combinations thereof.Apparatus of the invention can be implemented in a computer programproduct tangibly embodied in a machine-readable storage device forexecution by a programmable processor; and method actions can beperformed by a programmable processor executing a program ofinstructions to perform functions of the invention by operating on inputdata and generating output.

The invention can be implemented advantageously in one or more computerprograms that are executable on a programmable system including at leastone programmable processor coupled to receive data and instructionsfrom, and to transmit data and instructions to, a data storage system,at least one input device, and at least one output device. Each computerprogram can be implemented in a high-level procedural or object orientedprogramming language, or in assembly or machine language if desired; andin any case, the language can be a compiled or interpreted language.

Suitable processors include, by way of example, both general and specialpurpose microprocessors. Generally, a processor will receiveinstructions and data from a read-only memory and/or a random accessmemory. Generally, a computer will include one or more mass storagedevices for storing data files; such devices include magnetic disks,such as internal hard disks and removable disks; magneto-optical disks;and optical disks. Storage devices suitable for tangibly embodyingcomputer program instructions and data include all forms of non-volatilememory, including by way of example semiconductor memory devices, suchas EPROM, EEPROM, and flash memory devices; magnetic disks such asinternal hard disks and removable disks; magneto-optical disks; andCD_ROM disks. Any of the foregoing can be supplemented by, orincorporated in, ASICs (application-specific integrated circuits).

A number of embodiments of the invention have been described. Otherembodiments are within the scope of the following claims. Thus, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the invention.

1. A method comprises: converting acoustic energy representativeprincipally of a maternal heartbeat into a first electrical signal;converting acoustic energy representative of a maternal heartbeat and afetal heartbeat into a second electrical signal; processing the firstand second electrical signals to provide an electrical signalprincipally representative of the fetal heartbeat; and determining pitchperiods of the signal principally representative of the fetal heartbeat, wherein determining pitch periods comprises determining an initialperiod length value of the signal principally representative of thefetal heartbeat by finding a cepstrum of the first few pitch periods ofthe signal principally representative of the fetal heartbeat todetermine the frequency of the signal.
 2. The method of claim 1 furthercomprising: converting acoustic energy representative of maternaluterine contractions into a third electrical signal.
 3. The method ofclaim 1 wherein processing further comprises: rendering the electricalsignal principally representative of the fetal heartbeat on an outputdevice.
 4. The method of claim 1 further comprising: determiningprincipal components of determined pitch periods of the signalprincipally representative of the fetal heartbeat.
 5. The method ofclaim 1 further comprising: modulating the electrical signal principallyrepresentative of the fetal heartbeat with a signal in the audiblespectrum of human hearing.
 6. The method of claim 1 wherein determiningpitch periods further comprises: determining a beginning and endingpoint of each pitch period in the signal principally representative ofthe fetal heartbeat.
 7. The method of claim 6 wherein determining pitchperiods further comprises: determining a variation of time durationsbetween pitch periods; and using the length of a prior period as aninput to determine the duration of a subsequent pitch period.
 8. Themethod of claim 6 further comprising: applying principal componentanalysis to the determined pitch periods to compress data representingthe determined pitch periods.
 9. The method of claim 6 furthercomprising: processing the determined pitch periods to provide arepresentation; and compressing the representation of the determinedpitch periods; and storing the compressed representation of thedetermined pitch periods.
 10. A computer program product residing on acomputer readable medium for detecting fetal heartbeat energy, comprisesinstructions to cause a monitor to: convert acoustic energyrepresentative principally of a maternal heartbeat into a firstelectrical signal; convert acoustic energy representative of a maternalheartbeat and a fetal heartbeat into a second electrical signal; processthe first and second electrical signals to provide an electrical signalprincipally representative of the fetal heartbeat; determine pitchperiods of the signal principally representative of the fetal heartbeat; and determine an initial period length value of the signalprincipally representative of the fetal heartbeat by finding a cepstrumof the first few pitch periods of the signal principally representativeof the fetal heartbeat to determine the frequency of the signal.
 11. Thecomputer program product of claim 10 further comprising instructions to:convert acoustic energy representative of maternal uterine contractionsinto a third electrical signal.
 12. The computer program product ofclaim 10 further comprising instructions to: render the electricalsignal principally representative of the fetal heartbeat on an outputdevice.
 13. The computer program product of claim 10 further comprisinginstructions to: determine principal components of determined pitchperiods of the signal principally representative of the fetal heartbeat.14. The computer program product of claim 10 further comprisinginstructions to: modulate the electrical signal principallyrepresentative of the fetal heartbeat with a signal in the audiblespectrum of human hearing.
 15. The computer program product of claim 10further comprising instructions to: determine a beginning and endingpoint of each pitch period in the signal principally representative ofthe fetal heartbeat.
 16. The computer program product of claim 15further comprising instructions to: determine a variation of timedurations between pitch periods; and use the length of a prior period asan input to determine the duration of a subsequent pitch period.
 17. Thecomputer program product of claim 15 further comprising instructions to:apply principal component analysis to the determined pitch periods tocompress data representing the determined pitch periods.
 18. Thecomputer program product of claim 15 further comprising instructions to:process the determined pitch periods to provide a representation; andcompress the representation of the determined pitch periods; and storethe compressed representation of the determined pitch periods.
 19. Anapparatus comprises: circuitry to convert acoustic energy representativeprincipally of a maternal heartbeat into a first electrical signal;circuitry to convert acoustic energy representative of a maternalheartbeat and a fetal heartbeat into a second electrical signal;circuitry to process the first and second electrical signals to providean electrical signal principally representative of the fetal heartbeat;circuitry to determine pitch periods of the signal principallyrepresentative of the fetal heart beat; and circuitry to determine aninitial period length value of the signal principally representative ofthe fetal heartbeat by finding a cepstrum of the first few pitch periodsof the signal principally representative of the fetal heartbeat todetermine the frequency of the signal.
 20. The apparatus of claim 19further comprising: circuitry to convert acoustic energy representativeof maternal uterine contractions into a third electrical signal.
 21. Theapparatus of claim 19 further comprising: circuitry to render theelectrical signal principally representative of the fetal heartbeat onan output device.
 22. The apparatus of claim 19 further comprising:circuitry to determine principal components of determined pitch periodsof the signal principally representative of the fetal heartbeat.
 23. Theapparatus of claim 19 further comprising: circuitry to modulate theelectrical signal principally representative of the fetal heartbeat witha signal in the audible spectrum of human hearing.
 24. The apparatus ofclaim 19 further comprising: circuitry to determine a beginning andending point of each pitch period in the signal principallyrepresentative of the fetal heartbeat.
 25. The apparatus of claim 24further comprising: circuitry to determine a variation of time durationsbetween pitch periods; and circuitry to use the length of a prior periodas an input to determine the duration of a subsequent pitch period. 26.The apparatus of claim 25 further comprising: circuitry to applyprincipal component analysis to the determined pitch periods to compressdata representing the determined pitch periods.
 27. The apparatus ofclaim 25 further comprising: circuitry to process the determined pitchperiods to provide a representation; and compress the representation ofthe determined pitch periods; and store the compressed representation ofthe determined pitch periods.